Many of today's portable radio products are required to meet a Factory Mutual Intrinsically Safe rating. The purpose of this rating is to prevent ignition of an explosive atmosphere while operating electrical devices within such an environment. Sparking at connectors, for example, must be limited to sufficiently low energy that flammable atmospheres will not ignite. To guarantee meeting the intrinsically safe rating in today's radio products, an internal resistor is placed in series with the battery supply for the accessory power available at the accessory connector. While the resistor allows the radio to be intrinsically safe, it also limits the available power to an accessory, preventing the deployment of high power accessories, such as GPS and large displays with backlighting.
The universal serial bus (USB) interface was designed as a “hot-swappable” connector which enables peripherals to be connected to a host, such as a personal computer (PC), without powering down the system. The USB connector follows an industry standard for low power peripherals. The power contacts in the USB connector contact first and detach last, but the USB system does not guarantee turning off the power for purposes of preventing hazardous sparking.
Accordingly, there is a need for a means for a radio to supply higher power to an accessory across an accessory connector while still preventing sparks of sufficient energy to ignite a hazardous atmosphere.